The high boiling point hydrocarbon oils such as tar sand oil and petroleum distillation residue obtained by the atmospheric distillation or vacuum distillation of crude oil contain a large amount of impurities such as metals, sulfur compounds and nitrogen compounds. Thus, the hydrogenation technique as the most common purification means is now applied to the high boiling point hydrocarbon oils for utilizing them as various types of fuels and raw materials for chemical industry.
The trend toward a rapid shift of the stock oil to heavier oils is encountered especially in recent years. Accordingly, the demand for the resolution of technical problems of demetallization is increasing in addition to the problems of the conventional desulfurzation, denitrification and conversion to a light oil.
A problem of process technology in the hydrogenation of heavy oils is not only how to attain a high level of desulfurization degree but also how to remove metals as impurities and how to reduce the effect of metal deposition onto catalyst on the catalytic performance.
As different from sulfur compounds and nitrogen compounds, organometallic compounds and other metal components contained in the stock oil generally involve a phenomenon such that, except for unreacted part, substantially all of them deposit in the form of, for example, metal sulfides on the catalyst in accordance with the advance of the reaction and are not discharged outside the reaction system through the period of operation. This results in inviting problems such that the catalytic performance is lowered and that clogging of the catalyst bed increases the pressure drop in the reaction vessel.
Further, in the process of hydrogenation for heavy oil, carbonaceous substances (hereinafter may be referred to as "carbon") are deposited by the hydrocarbon decomposition reaction which is regarded as an inevitable side reaction.
The above deposited substance is the major cause of the pressure drop increase (hereinafter may be referred to as ".DELTA.P increase") experienced in the fixed bed reactor in which a particulate catalyst is packed.
In the process of heavy oil hydrogenation, generally, the type (for example, active metal species and amount of carried metals) of catalyst to be packed in the reactor and the amount of packed catalyst are determined on a condition that the catalyst would effectively function throughout the packed bed during the period of one cycle of continuous operation. However, in the actual operation, a rapid increase of .DELTA.P occurs before the planned volume of oil passage with the result that an arrival at the state of being no longer capable of continuing the operation is experienced.
That is, as long as the catalyst is appropriate for the stock oil, the activity of latter stage desulfurization catalyst is lowered in accordance with the predetermined schedule before the .DELTA.P increase of first direct desulfurization column (demetallizing reaction tower), and the operation is terminated. Unexpected deactivation of the desulfurization catalyst is caused by deterioration of the demetallizing catalyst at a latter period of the operation with the result that heavy metals contained in the stock oil acceleratingly deposit on the desulfurization catalyst. Moreover, at a latter period of the operation, the temperature of the catalyst bed is raised in accordance with the lowering of the activity of the demetallizing catalyst and desulfurization catalyst to thereby compensate for the lowering of the catalyst activity. However, this temperature increase causes conspicuous coking on the catalyst to thereby provide another cause of the deterioration of the desulfurization catalyst. Thus, since the catalyst deterioration and the catalyst bed clogging rapidly advance at a latter period of the operation, skilled operation management for the direct desulfurizer resides in terminating the scheduled operation just before the rapid advance of the catalyst deterioration and catalyst bed clogging. In this event, although a latter part of the demetallizing catalyst bed retains a demetallizing capability, the demetallizing catalyst bed as a whole has poor activity, so that discontinuation of the operation cannot be avoided. Reinforcing the amount of catalyst of the demetallizing catalyst bed may be contemplated as a countermeasure. In this case as well, the problem of clogging of the top part of catalyst bed at an inlet of the demetallizing catalyst cannot be solved, so that, after all, the operation period substantially cannot be prolonged because of the pressure drop increase although the demetallizing capability of the demetallizing catalyst bed as a whole is retained in large proportion due to the catalyst reinforcement.
The inventors studied the reaction mechanism of the catalyst layer based on the results of observation of catalyst having been used in an actual direct desulfurizer and the demetallizing reaction test in laboratory and recognized the occurrence of the following phenomenon on the catalyst.
Illustratively, in the initial period of the use of the demetallizing catalyst, demetallization is preferentially advanced in catalyst particles in which active metal species such as molybdenum are present and impurity metals such as vanadium and nickel contained in the heavy oil deposit and are retained in the catalyst particles.
When a large amount of vanadium and other impurity metals are accumulated in the catalyst particles with the passage of demetallization time, the active metals (hereinafter represented by molybdenum) within the catalyst particles gradually move to a surface layer of the catalyst particles to thereby form a concentrate layer and, further, a molybdenum layer is formed outside the catalyst particles. The molybdenum which is present in the outside layer of the catalyst particles retains catalytic activity and acts to deposit vanadium, nickel and iron of the heavy oil in the outside layer of the catalyst particles. When the demetallization time further lapses, the outside layer of the catalyst particles which is composed of molybdenum, vanadium, nickel, sulfur, iron, carbon precursor and the like expands, so that all the spacings between the demetallizing catalyst particles are buried by the catalytic action of the metals present in the layer. In this instance, the whole space of the demetallizing catalyst bed including the spacings between the catalyst particles is packed with a solid precipitate with the result that the pressure drop of catalyst bed is rapidly increased to thereby disenable operation of the direct desulfurizer.
In the process of hydrodemetallizing heavy oils, the resolution of the problem of the .DELTA.P increase of catalyst layer has been a longtime theme of the art. For example, U.S. Pat. No. 4,510,263 describes a catalyst packed in a reactor, which is a cylindrical extrudate that exhibits a .DELTA.P increase smaller than those of the catalysts of spherical and columnar extrudates, the above cylindrical extrudate having an internal wall provided with a rib or a vane of, for example, cruciform section so as to attain an improvement of mechanical strength and an expansion of active surface.
Japanese Patent Laid-open Publication No. 63(1988)-194732 proposed, as means for resolving the problem of clogging and activity deterioration, regulating the concentration distribution of active metal component in a catalyst support so that, in a cutting plane of the catalyst support, the concentration is maximized between the center and the outer surface of the catalyst support to thereby suppress the reaction at the outer surface of the catalyst and selectively give preference to the reaction at a region between the above center and outer surface with the result that the amount of deposition on the outer surface is reduced.
Further, Japanese Patent Laid-open Publication No. 2(1990)-305891 proposed structuring the surface of catalyst support particles so that the specific surface area is not greater than 1 m.sup.2 /g and the pore diameter is at least 10 .mu.m to thereby not only reduce the number of active sites on the outer surface of the catalyst and give preference to the reaction in pores but also, simultaneously, expand the volume for accommodation of deposited substance with the result that expansion of the catalyst volume is suppressed, thereby resolving the problem of clogging.
In the above process for hydrodemetallizing the stock oil (hydrocarbon oil), an economic advantage is realized by conducting a continuous operation for a prolonged period of time. However, the stock oil increasingly tends to become heavier. The inventors have extensively studied the prior art including the inventions described in the above literature and have found that, in the above process of hydrodemetallizing a heavy stock oil, conducting a continuous operation for a prolonged period of time is difficult with the use of any of the conventional catalysts. For example, with the use of the above catalyst described in U.S. Pat. No. 4,510,263, it is feasible to give a relatively large channel to the reaction fluid (hydrocarbon oil) but it is difficult to maintain the channel for a prolonged period of time.
An object of the present invention is to provide a hydrodemetallizing catalyst for a hydrocarbon oil which, in the hydrogenation of a hydrocarbon oil, especially, a heavy oil, prevents deposition of, for example, heavy metals between catalyst particles to thereby prevent clogging of the catalyst bed and maintain the space between catalyst particles so that a differential pressure increase is prevented with the result that the heavy oil can continuously be hydrodemetallized for a prolonged period of time and which further prevents sticking of catalyst particles to thereby facilitate the withdrawal of the catalyst after the termination of the operation. Another object of the present invention is to provide a process of hydrodemetallizing a hydrocarbon oil with the use of this hydrodemetallizing catalyst for a hydrocarbon oil.